This invention relates to a process for producing cyclohexane by benzene hydrogenation, and, more particularly, to a process for producing cyclohexane by benzene hydrogenation using a hydrogen source that contains impurities.
Over the years, researchers have developed numerous processes for manufacturing cyclohexane from the hydrogenation of benzene. For the most part, the majority of these various processes differ from each other in the techniques used to compensate for impurities, found in either the reaction components themselves or that are generated during the hydrogenation process.
For example, U.S. Pat. No. 3,711,566 (Estes et al.) describes a process in which aromatic hydrocarbon feedstocks containing sulfur are hydrogenated using a fluorided-platinum catalyst. Sulfur, a known poison to platinum catalysts, causes rapid deactivation of the catalyst as the hydrogenation process proceeds. Adding fluorine to the catalyst reduces sulfur poisoning; however, this undesirably increases hydrocracking activity that also deactivates the catalyst. Estes et al. inhibited hydrocracking activity by adding extremely small amounts, of carbon monoxide (a poison of metal catalysts itself) to the pure-hydrogen feed stream. This allowed the carbon monoxide to interact with the acidity of the fluorided-catalyst surface and prevent reactions, like hydrocracking, from taking place. Because carbon monoxide can also poison and deactivate the catalyst, care must be exercised in both purifying the hydrogen feed stream and in adding the carbon monoxide to the pure-hydrogen feed stream in order to achieve proper hydrogenation. This type of hydrogenation process therefore appears most useful when the hydrocarbon feedstock contains substantial amounts of sulfur requiring the catalyst to contain fluorine to prevent the sulfur from poisoning the catalyst.
U.S. Pat. No. 4,626,604 (Hiles et al.) describes a process in which hydrogenation occurs in a series of catalytic stages using at least three adiabatic reaction vessels. Because hydrogenation occurs in stages, lower operating temperatures can be used, which in turn reduces the formation of byproducts such as esters that can poison the catalysts and decrease, catalytic activity. However, Hiles et al. requires that the liquid unsaturated aromatic hydrocarbon be vaporized prior to mixing with the hydrogen gas. Portions of the vaporized unsaturated aromatic hydrocarbon are then hydrogenated in each catalytic stage before the saturated hydrocarbon is cooled and condensed back to liquid-form.
Of particular concern in a conventional hydrogenation of benzene process are impurities found in the hydrogen source, because such impurities often deactivate the catalyst used to promote the hydrogenation reaction. Carbon monoxide is one such impurity that can reversibly poison catalysts, like nickel, used in benzene hydrogenation processes. In the poisoning process, carbon monoxide is adsorbed onto the active sites of the nickel catalyst surface, thereby reducing the activity of the catalyst. Depending on the concentration of carbon monoxide in the hydrogen source, the nickel catalyst can rapidly deactivate.
Once the nickel catalyst has deactivated, the catalyst may be regenerated by heating the catalyst at a temperature from about 220xc2x0 C. to about 260xc2x0 C. Because this regeneration process may not be completed in the presence of benzene or cyclohexane (the temperatures required for regeneration tend to promote the formation of large quantities of undesirable cracking products), the reactor must be taken off-line before regeneration of the catalyst. Due to the obvious inconvenience associated with taking the reactor off-line, most conventional benzene hydrogenation processes are designed to limit or prevent deactivation of the catalysts.
In order to prevent or limit deactivation of the nickel catalysts commonly used in benzene hydrogenation processes, most conventional processes require that a highly pure hydrogen source be used. Relatively pure hydrogen sources may be obtained from a steam reformer, and such hydrogen streams typically contain about 96 mole % hydrogen, about 4 mole % methane, and less than about 10 ppm of carbon monoxide and other impurities. Even with such low carbon monoxide levels, these hydrogen streams must still often be further purified to reduce the carbon monoxide levels to less than about 1 ppm before use. As such, these hydrogen streams tend to be expensive, yet they are frequently used because no other alternatives have been available.
Less pure sources of hydrogen are available from steam cracking, catalytic reforming, and hydroalkylation. Hydrogen streams obtained from these sources typically contain from about 10 mole % to about 80 mole % hydrogen, with the remainder comprising impurities such as methane, other light hydrocarbons, and/or carbon monoxide. The level of carbon monoxide in hydrogen streams from these sources is often as great as about 5000 ppm, which often prevents the use of these hydrogen sources in conventional benzene hydrogenation processes.
Therefore, what is needed is a process that: (i) promotes the hydrogenation of benzene to cyclohexane that operates using a lower purity, and thereby, a less expensive source of hydrogen; (ii) proceeds without deactivation of the catalyst due to the presence of carbon monoxide or other impurities in the hydrogen source; and (iii) promotes the hydrogenation of benzene without contributing to the formation of a significant amount of cracking products, such as methylcyclopentane.
The present invention, accordingly, provides for a process of producing cyclohexane by benzene hydrogenation using a hydrogen source that contains impurities. The supported catalysts used in the present invention reduce benzene to cyclohexane, and reduce carbon monoxide to methane and water. Alkenes, such as ethylene, are also reduced to their alkane counterparts. An advantage of the present invention is that the catalysts used in the disclosed process, if used under the reaction conditions disclosed, do not deactivate in the presence of carbon monoxide or other impurities typically found in hydrogen sources. Another advantage is that the disclosed process proceeds without the formation of a significant amount of cracking products, such as methylcyclopentane.
The present invention provides for a process for causing the simultaneous production of cyclohexane from the hydrogenation of benzene and chemical reduction of certain impurities that may be present in the reactants. The process involves providing a first stream comprising benzene; providing a second stream comprising hydrogen and impurities; mixing the first and the second streams to form a reactive mixture; and contacting the reactive mixture with a catalyst to effectuate the reduction of the benzene and impurities. Under the preferred reaction conditions of the present invention, the catalyst will not deactivate rapidly, high benzene and hydrogen conversions will be obtained, and cracking product formation will be held within acceptable limits.
The hydrogen stream used in the process of the present invention may be obtained from a variety of sources, including, but not limited to, steam cracking, catalytic reforming, and/or hydroalkylation. Preferably, the hydrogen source should contain no more than about 15 mole % of impurities, such as, but not limited to, carbon monoxide or light hydrocarbons. More preferably, the hydrogen source should contain no more than about 5 mole % of carbon monoxide and about 10 mole % of light hydrocarbons. The light hydrocarbons may comprise alkanes and/or alkenes with from about one to about three carbon atoms, including, but not limited to, methane and/or ethylene.
The benzene stream may be obtained from any number of sources, including, but not limited to hydrodealkylation, pyrolysis, catalytic reforming or fractional distillation.
The catalysts used in the process of the present invention may be prepared according to any suitable technique known in the art. Typically, the catalyst comprises nickel and copper. The catalyst may also optionally comprise chromium, manganese, iron, cobalt, zinc, molybdenum, tin, or combinations thereof. Preferably, the catalyst comprises from about 15 weight % to about 35 weight % nickel, from about 1 weight % to about 15 weight % copper, and from about 0 weight % to about 5 weight % chromium, manganese, iron, cobalt, zinc, molybdenum, tin, or mixtures thereof. More preferably, the catalyst comprises from about 22 weight % to about 28 weight % nickel, from about 2 weight % to about 6 weight % copper, and from about 0 weight % to about 3 weight % chromium, manganese, iron, cobalt, zinc, molybdenum, tin, or mixtures thereof The support for the catalyst may comprise any material suitable for a support. Preferably, the support comprises either alumina or silica.
Surprisingly, the catalysts used in the process of the present invention do not lose catalytic activity in the presence of impurities, including carbon monoxide, that are contained in the hydrogen source. Nickel catalysts are notoriously well known in the art to be poisoned by carbon monoxide, as discussed above. The process of the present invention prevents this poisoning while reducing benzene to cyclohexane at sufficient rates. Also surprisingly, the catalysts not only function to reduce benzene to cyclohexane, but also reduce carbon monoxide to methane and water, and alkenes, such as ethylene, to their alkane counterparts. Unexpectedly, the reduction of benzene and the impurities proceeds with minimal formation of cracking products, such as methylcyclopentane, even at increased temperatures.
In order to practice the process of the present invention, any suitable reaction vessel may be used. Preferably, the reaction vessel is a reactor. More preferably, the reaction vessel is a jacketed, stainless steel, tubular reactor.
The process of the present invention should be conducted under conditions sufficient to promote the reduction of benzene and the impurities in the reactive mixture. It will be understood by those skilled in the art that conditions of temperature and pressure may vary depending on other variables such as the desired conversion, benzene concentration, hydrogen concentration, carbon monoxide concentration, catalyst particle size, catalyst composition, the heating/cooling efficiency of the reactor system, etc.
Generally, during operation, the exotherm or hot spot temperature in the reactor should be maintained above about 160xc2x0 C. Preferably, the exotherm or hot spot temperature in the reactor should be maintained from about 160xc2x0 C. to about 340xc2x0 C. More preferably, the exotherm or hot spot temperature in the reactor should be maintained from about 190xc2x0 C. to about 280xc2x0 C.
Generally, the reactor pressure should be maintained above about 50 psig. Preferably, the reactor pressure should be maintained from about 250 psig to about 2500 psig, and more preferably, from about 400 psig to about 800 psig.
In the process of the present invention, it is preferable to use an excess of benzene, relative to the amount of hydrogen. Under such conditions, the process should be used in, a xe2x80x9cfront-endxe2x80x9d reactor (ie. where the reactor is the first reactor in a series of reactors). The resulting product may then be xe2x80x9cfinished offxe2x80x9d (ie. the benzene levels may be reduced to ppm levels) by a subsequent reactor.
The final cyclohexane product can be collected by separation means generally used in separating liquids such as distillation, centrifugation, density differences or chromatography.
While it is the preferred method to use an excess of benzene in carrying out the process of the present invention, it would be obvious to one skilled in the art that an excess of impure hydrogen, relative to the amount of benzene, could also be used with the catalysts and conditions disclosed in the present invention.
The following examples are illustrative of the present invention, and are not intended to limit the scope of the invention in any way.
Catalyst A is a conventional, highly active nickel on silica catalyst, available from the Engelhard Corporation, Beachwood, Ohio, under the name Ni-5256 E {fraction (3/64)}. It contains 57% nickel, has a surface area of 260 m2/g, a total pore volume of 0.5 cc/g, and was in the form of {fraction (3/64)} inch diameter extrusions. Before use, the catalyst was reduced and stabilized. This catalyst is recommended by the supplier for use in benzene hydrogenation processes.
Catalyst B is a 24% nickel and 4.5% copper catalyst on an alumina support. It was prepared using the standard technique of impregnation of a formed alumina support ({fraction (1/16)} inch extrusion, surface area 100 m2/g) with an aqueous solution of nickel and copper nitrates. The wet impregnated support-was dried in an oven to remove the water, and then calcined at about 400xc2x0 C. to decompose the nitrates to the corresponding nickel and copper oxides. The catalyst precursor was then xe2x80x9cactivatedxe2x80x9d by reaction with hydrogen, at temperature of about 300xc2x0 C. Following activation, the catalyst was stabilized to air with dilute oxygen, at a low temperature. The final catalyst had a surface area of 68 m2/g, and a pore volume of 0.40 cc/g.
Catalyst C was prepared by co-precipitating a mixture of nickel, copper, and chromium carbonates from an aqueous solution of the mixed metal nitrates and sodium carbonate. The precipitated mixture was then washed with fresh water, dried, and then calcined to produce an oxide powder. The oxide powder was then compounded with fine gamma alumina powder, and the resulting product was formed into {fraction (1/16)} inch diameter extrusions. The extrusions were then dried, calcined, activated by reaction with hydrogen, and then stabilized to air by partial reoxidation with air under controlled conditions. The catalyst contained 26.3% nickel, 3.9% copper, and 0.92% chromium. The catalyst had a surface area 209 m2/g, and a pore volume of 0.46 cc/g.
The reactor was a Dowtherm-jacketed 1.338xe2x80x3xc3x9730xe2x80x3 stainless-steel tube equipped with a xc2xcxe2x80x3 thermowell running up through the center of the tube. For each run, the reactor was filled with 250-mL of a catalyst (either catalyst A, B, or C), with glass beads added at the top and bottom of the catalyst.
Liquid benzene (20 wt. %)/cyclohexane (80 wt. %) and gaseous hydrogen (25 mole %)/ methane (75 mole %), with and without various impurities, were fed into the reactor through a static mixer. The reactor pressure was maintained at about 500 psig by means of a backpressure regulator. Both liquid and gaseous samples were collected in stainless-steel bombs and analyzed by a gas chromatograph.